>John's comment gave me a scare when I realised that one of my designs >uses two directly cascaded '595s. This seems to be a drop-off in the >design of this part as the Philips data sheet for the HC/HCT595 states >that the Q7' (QH') output is intended for cascading - and yet it is >clear from closer examination that a skew of 10ns or more between the >shift clocks of the respective devices could result in marginal timing >and possibly incorrect operation. As John says, an extra flip-flop >acting on the falling edge of the shift clock would solve this. > >Having examined my own design, it looks as if I can get away with this, >as the two '595s in question are driven directly by a PIC output via a >short PCB trace with no additional loading. The data sheet suggests >that, when operating at +5V, there is a positive margin of 10ns worst >case (still not a lot!) between the propagation delay (from shift clock >to Q7') of the first '595 and the data set-up plus hold time for the >second '595. However, this margin becomes negative at lower supply >voltages - so I'm glad that my application isn't safety-critical! You don't have a problem, and you can't add the flip flop to fix the problem you don't have because you can't get access to the shift register stages before the latch! The '595 is DESIGNED to be cascaded. You are right - there is little room for timing error. If you need to feed the Qs output off-board to another '595, you will probably run into problems. The cure is simple - add a RC delay to the clock line feeding the off-board shift registers. My standard off-board I/O connections always include series resistors for 'OOPS' protection. They often also have small capacitors for EMI protection. You will see that this has lead to timing problems for cascaded shift registers! The cure was as mentioned above - ensure the RC delay on the clock line is longer than any of the other delays (but shorter than your clock period!) and the problem does not exist. Note: my off-board shift registers operate at very slow data rates (512 bits / sec) for EMI reasons and I have LOTS of leeway to ensure that setup times are ample. Dwayne Reid Trinity Electronics Systems Ltd Edmonton, AB, CANADA (403) 489-3199 voice (403) 487-6397 fax